iii
Molecular Dynamics computer simulations are used to investigate the structure
and dynamics of nanoscopically confined polymers intercalated in layered-inorganic hosts
to form nanocomposites. The focus of this research is on understanding how the nature
of the organic film is influenced not only by the severe confinement, but also by the
interactions of the organic film with lithium counterions present in the slit pore.
Molecular Dynamics simulations are performed using atomistically detailed models
for interactions between various species in the system ; the simulation setup and
potentials used mimic the experimental system of poly(ethylene oxide) confined between
mica-type montmorillonite clays, i.e. 2:1 alumino-silicates. The force fields employed
were well tested in earlier simulations of Li+/PEO interactions, and in studies of
PEO/Montmorillonite nanocomposites.
Simulations were also performed on bulk polymer to comparatively contrast the
differences in behavior between bulk and confined PEO. For the bulk Li
+/PEO system,
it is seen that there is a drastic change in the structure as explored by three different
order parameters, at a temperature close to the melting point of bulk PEO. In sharp
contrast, the structure of confined PEO exhibits no dramatic qualitative change with
temperature. Instead there is only a gradual quantitative difference in the structural
ordering, which reveals that the confined organic film exists in the same amorphous
state throughout the temperature range of study.
iv
In addition to the structure, the dynamics of the confined systems are markedly
different from what one would normally expect for a regular composite. Unlike the bulk
system, which exhibits clear solid-like and liquid-like polymer motion below and above
the experimental melting point of PEO, there seems to be no distinct change in dynamics
of the confined polymer as probed experimentally by 2H NMR. This anomalous behavior
was reproduced in our simulations, and the coexistence of fast and slow relaxation times
for C-H bond reorientations was attributed to the presence of adjacent Li
+, density
inhomogeneities that were stabilized in the confinement, and enhanced translational
motion at all temperatures. Finally, the temperature dependence of Li
+ diffusion in
bulk PEO shows two distinct mechanisms of motion, for regions below and above the
melting point of the polymer. For the nanocomposite, a single mechanism for lithium
ion transport at all temperatures was identified. It is revealed that enhanced polymer
dynamics in the confinement are responsible for the higher diffusion coefficient coefficient
for Li+ in PEO/MMT compared to bulk PEO, at lower temperatures.